This invention relates to a system to alert an aircraft pilot of the presence and general location of other aircraft that might constitute a collision threat to the pilot's aircraft. In particular, the invention relates to a system that analyzes transponder signals from the other aircraft to determine the presence, direction, distance, and altitude of a potential collision threat relative to the pilot's aircraft and does so passively, i.e., without adding to the transmissions that already crowd that part of the radio frequency band.
The avoidance of midair collisions is primarily the responsibility of the pilots of the aircraft that might be involved, and it is appropriate that this be so since the pilot has the most immediate concern about the possibility of such an event. The Federal Aviation Agency (FAA) does maintain radar and communications systems that enable operators on the ground to advise pilots of the presence, general location, and altitude of other aircraft, particularly in the vicinity of major airports. However, the advice is not always available and complete. Pilots may be advised to avoid proceeding in one direction because of the danger of collision, only to find that the area toward which they are directed has other dangers.
There are also current discussions on expanding the current system in a way that would have a stored program tell the pilot the proper evasive action to take. Such programs are based on limited assumptions that may not be appropriate for real life situations, and it is much to be preferred that the pilot be presented with all immediately relevant information concerning all other nearby aircraft so that the pilot can decide on the best course of action. Insofar as other aircraft are concerned, the pilot needs to know the distance, direction, and altitude of those aircraft relative to the pilot's aircraft. While such information could be obtained if the pilot's aircraft had radar equipment on board, a great many owners cannot afford the expense, and their aircraft could not carry the equipment.
The Federal Aviation Agency (FAA) maintains an Air Traffic Control Radar Beacon System (ATCRBS) in which primary surveillance radar stations monitor the location of aircraft in their vicinity by transmitting a burst of microwave energy about 400 times per second from a highly directional antenna that focuses almost all of the energy in a narrow, wedge-shaped beam only about 1.degree. wide. The antenna is rotated about once every 4.3 seconds, and when the beam sweeps through the part of the sky in which an aircraft is flying, a tiny fraction of the energy from each burst strikes the aircraft, which reflects it in all directions. Only a tiny fraction of the reflected energy is directed back to the antenna of the surveillance station, where it is picked up and utilized by circuits in the receiving section. The reflected pulse signals received at the surveillance station are detected and fed to a plan position indicator where they are displayed by pulsing the beam of a cathode-ray tube to generate a pattern of bright spots on the screen, corresponding to the locations of aircraft around the station. Reflections from large metal aircraft, especially those not too far from the station, produce relatively bright spots on the screen of the c.r.t., but the spots produced by echoes from smaller aircraft, particularly those not made of metal, and from distant aircraft, are not so bright.
It was realized long ago that the weak reflected signals received by the radar ground stations made it difficult to keep accurate track of the aircraft, even those that reflected signals relatively effectively. Consequently, a system was devised to enable ground stations to send out encoded, interrogating signals and to enable each aircraft equipped to do so to transmit responsive, encoded pulse signals. The apparatus installed in the aircraft to receive and analyze the ground station's signal and to transmit a response to it is called a transponder. The response, while transmitted omnidirectionally, is many orders of magnitude more powerful than a radar reflection signal. Since the signal transmitted by the aircraft's transponder is received by the ground station about 3 .mu.sec after the echo signal, but while the antenna of the ground station is still aimed in the direction of that aircraft, the equipment at that station can use the transponder signal to enhance or replace an otherwise weak presentation of an echo signal. Furthermore, the highly directional antenna used by the ground station in this improved system keeps the station from receiving many interfering signals, and it is relatively easy to separate the desired signals received at any instant from the undesired ones received simultaneously.
The radar station actually includes two radar systems. The one that transmits the narrow 1.degree. beam and simply displays a brightened spot representing the echo is called the primary system. The other, which causes the transponder in the aircraft to generate its own responsive signal, is called the secondary surveillance radar (SSR) system. Antennas for both of the ground station's systems rotate in unison, but the SSR antenna is not as directive as that for the primary radar and sends out a wedge-shaped beam about 4.degree. wide transmitted on a carrier frequency of 1030 MHz and repeated about 400 times per second. One such group, called Mode A, consists of two pulses, each 0.80 .mu.sec long and with the leading edge of the second pulse 8 .mu.sec behind the leading edge of the first. When a Mode A interrogation signal is received by a transponder receiver, it causes the transponder to generate a Mode A response signal consisting of pulses encoded with the identification assigned to that aircraft at least for that flight and transmitted on the transponder frequency of 1090 MHz.
Another interrogation signal, called a Mode C signal, transmitted by the SSR system on the same 1030 MHz interrogating frequency, consists of two pulses with the leading edge of the second 21 .mu.sec behind the leading edge of the first. This causes the aircraft's transponder to transmit, on its 1090 MHz carrier, a Mode C response signal consisting of pulses encoded to represent the aircraft's altitude, if the aircraft is properly equipped with an encoding altimeter. All aircraft flying within 30 miles of major airports are required to have operating transponders so equipped. Still other groupings of SSR pulses are in use or have been proposed for eliciting other information from the aircraft's transponder.
The Mode A identification signal transmitted by a transponder consists of two framing pulses, which have leading edges spaced 20.3 .mu.sec apart, and from 0 to 12 pulses, or bits, arranged in four groups of three bits each at specific times between the framing pulses. All of these response pulses, i.e., the framing pulses as well as the encoding pulses, are identical in amplitude and in duration, which is 0.45 .mu.sec. There are actually 13 equally spaced time slots for encoding pulses between the framing pulses, but the seventh slot is always empty. The pulses constitute information bits, and the system is set up so that, if a pulse is transmitted at one of the assigned times, the system will treat it as a 1 bit, while if no pulse is sent by the transponder at that time, it will be treated as a 0 bit. The total number of identifications possible with a binary signal having twelve bits, as this complete signal does, is 4096. However, certain numbers are reserved for special purposes in the SSR system, so the total available for use is less than the theoretical maximum.
The Mode C interrogation causes a properly equipped transponder to send a binary signal encoded to represent the pressure altitude of the aircraft, based on a standard barometric pressure of 29.92" of mercury. The Mode C response signal for altitudes below 30,800' uses only nine bits, which makes it possible to identify 512 different altitudes. At low altitudes, starting at -1000' (which can occur even at actual altitudes above sea level, due to the fact that the measurement is based on a standard altimeter setting of 29.92" of mercury), the system identifies altitudes at 100' intervals. The nine bits used for transmitting altitude information are identical in every way with nine of the twelve bits used for transmitting identification information, which means that 512 of the 4096 possible identification codes are indistinguishable in form from altitude codes.
Transponder receivers are tuned to the 1030 MHz signal from the ground stations, not to the 1090 MHz frequency transmitted by transponder transmitters, but even if an aircraft did carry a receiver tuned to that frequency, enabling it to receive signals from transponders of other aircraft within the range of the receiver, the pilot would not be able to tell, from the mere reception of a second aircraft's transponder signal, whether a given pulse train received from the transponder of another aircraft represented an identification signal or an altitude signal. The test would have to be a negative one, because any transponder signal that could be an altitude signal could also be an identification signal, but the reverse is not true. In addition, altitude signals above 30,800' use more than nine bits, and many jets fly higher than that.
Ground station apparatus has no difficulty in determining whether a received transponder signal is a Mode A identification signal or a Mode C altitude signal, since the station apparatus recognizes each transponder response as being a response to the type of interrogating signal the ground station has just transmitted. In addition, ground stations receive transponder responses almost exclusively from aircraft toward which the highly directive SSR antenna of the ground station is pointed.
Apparatus in one aircraft receiving transponder signals from all other aircraft within receiving range has neither of those advantages.
However, ground stations always use a standard sequence of interrogation: they send out two Mode A signals requesting information as to the identification of any aircraft receiving those signals, followed by one Mode C signal requesting information as to the altitude of that aircraft. This makes it possible for any aircraft having a receiver incorporating the analytical system of this invention to compare three successive groups of pulse signals from the transponder of the second aircraft and tell both the identification and the altitude of the other aircraft. An exception occurs during "squawk ident" when the pilot is requested by the ground station operator to cause the transponder to transmit only the identification code for a 20 second interval. If the altitude code is the same as the assigned identification code, it would not matter which was considered to be the altitude code.
If the second aircraft is within a predetermined number of feet of the altitude of the pilot's aircraft, it is necessary for the pilot to know in which direction to look for the other aircraft. Two aircraft flying at the same altitude may be far enough apart to allow some time to take appropriate evasive action, or they may be following diverging courses that will never cause them to collide. However, even if those aircraft are several miles apart, and are not headed directly toward each other, they can still be heading toward a collision. That will be true if they are both heading toward the same point in space and are holding a fixed bearing relative to each other, even if they are flying at different speeds. The pilot of the aircraft having the apparatus described herein would be able to recognize those conditions and would know where to look for the other aircraft. As a result, the pilot would be able to take appropriate evasive action as well as being able to notify the ground station of the existence of the other aircraft.